The present invention relates to an optical coupler/splitter for use in an optical fiber communications system or an optical sensor system, and more particularly, to an optical coupler/splitter with a filter, which makes it possible to locate a fault taking place in a system, especially in an optical fiber beyond the optical coupler/splitter and which features a simple structure, permitting high productivity.
It is known that an OTDR (optical time domain reflectometer) based on the backscattering method is used to locate a fault of deteriorated mechanical characteristic or transmission characteristic of an optical fiber in an optical fiber communications system or the like. In a typical OTDR, an optical output pulse from a semiconductor laser is guided into an optical fiber to be measured, and the back scattering light, which is scattered and bounced back from the optical fiber is detected, thus measuring a loss of the optical fiber.
However, the optical fiber communications system incorporates an optical coupler/splitter for branching optical power from a single input to a plurality of outputs or for combining a plurality of inputs into a single output. This occasionally causes difficulties in locating a fault point in the system, especially in an optical fiber beyond the optical coupler/splitter, by using a light signal from the OTDR.
For instance, it is difficult to locate a fault point in a system shown in FIG. 1, which system includes a 1-input, 4-output optical coupler/splitter 1 comprising an input port 1a and four output ports 1b, 1c, 1d, and 1e which are branched from the above-mentioned input port and to which optical fibers 3a, 3b, 3c, and 3d are connected. More specifically, when an attempt is made to locate the fault point in the system of FIG. 1, especially the fault point in the optical fiber 3a, 3b, 3c or 3d beyond the optical coupler/splitter 1 by using an OTDR 4a, which issues light of a fixed wavelength, from the side of an optical fiber 2 connected to the input port 1a, it is possible to determine a distance between the fault point in the optical fiber 3a, 3b, 3c or 3d beyond the coupler/splitter 1 and the OTDR 4a, but it is impossible to specify which optical fiber has developed the fault.
In addition, the distance between the OTDR 4a and the fault point can be determined provided that the coupler/splitter has a small number of branched optical ports, e.g., a combination of 1 input and 2 outputs or of 1 input and 4 outputs. In a coupler/splitter with more branched optical ports as in a coupler/splitter which has, for example, 1 input and 16 outputs, it becomes difficult to determine the distance between the OTDR and the fault point because interference light adversely affects the S/N ratio.
To resolve the difficulty described above, a system shown in FIG. 2 has been proposed. This system differs from the system shown in FIG. 1 in that it has an OTDR 4b which uses a light source of a variable wavelength in place of the OTDR 4a which issues light of the fixed wavelength illustrated in FIG. 1 and in the respective output ports of its coupler/splitter 1 are equipped with filters 5a, 5b, 5c and 5d which respond to different transmitting wavelengths or reflecting wavelengths and which generally have different transmissivities. According to the system, it is possible to determine the distance between the OTDR 4b and the fault point and also specify an output port which incurs the fault.
However, in the system illustrated in FIG. 2, it is necessary to provide all branched output ports of the coupler/splitter with different types of filters, requiring an extremely complicated manufacturing process with resultant poor productivity.